In manufacturing systems used for manufacturing semiconductor devices, for example, a vacuum apparatus employs seals such as O-rings for maintaining a chamber thereof at a given degree of vacuum, or the vacuum apparatus or another apparatus employs the seals for sealing an atmosphere in the chamber thereof off from the outside.
Preferred as such a seal is one which is made by molding a sealing composition into a prescribed shape and subjecting the molded article to a crosslinking reaction, the sealing composition comprising a polyallyl compound and an organic peroxide, as a crosslinking agent, and a fluoroelastomer capable of being crosslinked using the crosslinking agent.
By virtue of the properties of fluoroelastomer, the aforesaid seal exhibits high heat resistance when used in a manufacturing apparatus wherein an operating temperature is raised to above 200° C. Furthermore, the seal is also excellent in resistances to particular atmospheres in the manufacturing apparatus, such as low temperature plasma and etching gases. In addition, both of the fluoroelastomer and the crosslinking agent that form the seal are free from metals constituting a causative factor of the contamination of semiconductor devices and from sulfur which may deteriorate the nature of conductor circuits.
Accordingly, the aforesaid seal is suited for use as a sealing material in the apparatuses for manufacturing the semiconductor devices or the like.
The sealing composition as the material of the seal is admixed with a reinforcing material for controlling the hardness of the seal.
As the reinforcing material, fluorine resin powders have been commonly used, which are free from metals and sulfur and have good heat resistance (see, for example, Japanese Patent Publication No. JP-2937302-B).
However, the fluorine resin powders have low ability to serve as the reinforcing material. In order to control the hardness of the seal to a predetermined hardness level, therefore, the fluorine resin powder need be admixed to the sealing composition in an increased packaging percentage (wt % based on the overall weight of the composition including the crosslinking agent and such).
As for an example of a combination of a fluoroelastomer and a fluorine resin powder to be described hereinlater in a comparative example, the combination requires such a large amount of fluorine resin powder as 80 parts by weight based on 100 parts by weight of fluoroelastomer (packing percentage of 43.1 wt %) to be admixed in order to achieve Type-A durometer hardness of at least 70 A as determined in accordance with JIS K6253-1993 “Hardness Testing Method for Vulcanized Rubber” (equivalent to ISO 7619-1986).
However, it is impracticable to uniformly disperse such a large amount of fluorine resin powder in the sealing composition and hence, a fear exists that the fluorine resin powder may be unevenly dispersed. In the event of the unevenly dispersed resin powder, the physical properties of seals formed from the sealing composition containing unevenly dispersed resin powder are varied in one seal or among two or more seals.
Furthermore, because of the high content of fluorine resin powder, the resultant seals suffer such a large compression set percentage that the seals are susceptible to permanent set associated with compression in use. Consequently, the seals detrimentally have short service lives.